Method of preparing a polysilazane

ABSTRACT

A method of preparing an organosilazane including reacting a first halosilane having the formula RR 1  SiX 2 , wherein X is F, Cl, Br, or I, and each R and R 1 , independently, is X, H, or a lower alkyl group; a first primary amine compound having the formula R 2  NH 2 , wherein R 2  is H or a lower alkyl group; and a second primary amine compound having the formula R 3  NH 2 , wherein R 3  is H or a lower alkyl group, R 3  being different from R 2 , to form the organosilazane.

This is a divisional of co-pending application Ser. No. 07/237,515 filedon Aug. 26, 1988, now U.S. Pat. No. 4,983,552, which is acontinuation-in-part of Gallo, U.S. Ser. No. 06/811,483, filed Dec. 20,1985, now abandoned.

This invention relates to preparing organosilazanes.

Organosilazanes are organic compounds containing a backbone of siliconand nitrogen atoms, which can be polymerized to form polysilazanes.Polysilazanes are useful as ceramic-precursor polymers, forming uponpyrolysis, the ceramic materials silicon carbide and silicon nitride.

SUMMARY OF THE INVENTION

In general, the invention features a method for preparing anorganosilazane involving reacting a first halosilane having the formulaRR¹ SiX₂, wherein X is F, Cl, Br, or I, and each R and R¹,independently, is X, H, or a lower (1 to 3 carbon atoms) alkyl group; afirst primary amine compound having the formula R² NH₂, wherein R² is Hor a lower alkyl group; and a second primary amine compound having theformula R³ NH₂, wherein R³ is H or a lower alkyl group, R³ beingdifferent from R², to form the organosilazane.

In preferred embodiments, R² is H and R³ is a lower alkyl group, mostpreferably CH₃ ; the first halosilane is a trihalosilane; and the firsthalosilane is reacted with the first primary amine compound to form areaction product, which is then reacted with the second primary aminecompound to form the organosilazane. Thus, the preferred method involvessequential addition of the primary amine compounds.

In other preferred embodiments, there is reacted with the firsthalosilane, first primary amine compound, and second primary aminecompound a second halosilane different from the first halosilane andhaving the formula R⁴ R⁵ R⁶ Si_(x), wherein X is F, Cl, Br, or I, andeach R⁴, R⁵, R⁶, independently, is X, H, or a lower alkyl group.Preferably, the composition of the two halosilanes and their mole ratioare selected so that the theoretical nitrogen to silicon ratio in thepolymer formed from the organosilazane is 1.33. When the firsthalosilane is a trihalosilane (preferably CH₃ SiCl₃ or HSiCl₃) and thesecond halosilane is a dihalosilane (preferably CH₃ SiHCl₂ or H₂ SiCl₂),the mole ratio of the first halosilane to the second halosilane is about2:1. When the first halosilane is a dihalosilane (preferably CH₃ SiHCl₂or H₂ SiCl₂) and the second halosilane is a tetrahalosilane (preferablySiCl₄), the mole ratio of the first halosilane to the second halosilaneis also about 2:1.

The invention provides a simple and inexpensive method for preparingorganosilazanes. The organosilazanes thus prepared are thermally stableat ambient temperatures and resist hydrolysis by atmospheric moisturefor prolonged periods of time. In addition, they are readily polymerizedunder low temperature polymerization conditions (preferably in an inertatmosphere at temperatures between about 200°-300° C.) to polysilazaneswhich are stable, fusible, and soluble in organic solvents, and whichprovide good yields of silicon nitride----silicon carbide uponpyrolysis. Preferably, pyrolysis is carried out in an inert atmosphereat temperatures between about 250°-1200° C. The pyrolysis products arethermally and oxidatively stable at high temperatures (>1000° C.) andcan be used as protective coatings on fibers, e.g., carbon fibers.

Other features and advantages of the invention will be apparent from thefollowing description of the preferred embodiments thereof and from theclaims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

We now describe the synthesis and use of preferred organosilazanes ofthe invention.

In general, the synthesis involves reacting a halosilane with a primaryamine compound, and then reacting the product of that reaction with asecond primary amine compound different from the first. The synthesiscan be represented as follows: ##STR1##

Since the final organosilazane contains R, R¹, R², and R³, the molecularweight (and therefore the viscosity) of the organosilazane can becontrolled by choosing the proper halosilane and primary amine startingmaterials. In addition, one can control the position of the R, R¹, R²,and R³ groups in the final organosilazane, e.g., if the two amines usedare ammonia and methylamine, the methyl group will be bonded to thenitrogen atom of the organosilazane backbone if the methylamine is firstreacted with the halosilane, and to the nitrogen atom not in thebackbone if the ammonia is first reacted with the halosilane. Thepreferred reaction order is ammonia, followed by CH₃ NH₂. Since thehalogen atoms do not appear in the final organosilazane product, thechoice of halogen is not crucial, but is governed primarily by theconsideration of cost; chlorine is thus the preferred halogen.

The first step in the above-illustrated synthetic scheme is to dissolvethe halosilane in a dry, inert, organic solvent such as toluene,tetrahydrofuran, or diethylether, such that the halosilane is present inthe solvent at a concentration of 0.1 to 3.0 moles/liter. The solventdoes not participate in the reaction, and the solvent requirements areonly that it be capable of dissolving at least 0.1 moles ofhalosilane/liter and that it not interfere with the reaction.

The next step is to react the solubilized halosilane with a primaryamine R² NH₂. In this step, the mole ratio of halosilane: amine isbetween 10:1 and 1:2; higher ratios generally yield material having ashorter N-Si spine, and thus lower viscosity and molecular weight. Theamount of the first primary amine used should be low enough to ensurethat the reaction product contains some reactive halogen atoms availablefor replacement by R³ NH of the second primary amine. The reaction iscarried at a temperature between about 20° and 60° C., for a period of10 to 200 minutes. If the primary amine is a gas at the reactiontemperature, it is bubbled through the solubilized halosilane; if it isa liquid, it can be added dropwise to the halosilane.

The next step is to react the product of the halosilane-amine reactionwith the second primary amine. In this step, enough of the secondprimary amine is added so that all of the silicon-containing reactant isused. The reaction is carried out at a temperature of 20° to 60° C., fora period of 20 to 500 minutes. As in the case of the first primaryamine, the second amine is bubbled through or added dropwise to thefirst reaction product. The organosilazane product is recovered, e.g.,by filtering, and any remaining solvent is removed, e.g., by heating.

A particular organosilazane, of the formula ##STR2## was synthesizedfrom CH₃ SiCl₃, NH₃, and CH₃ NH₂, as follows (these compounds are allcommercially available).

CH₃ SiCl₃ (4.5 kg) was added with stirring to approximately 22 gallonsof dry toluene in a glass reactor fitted with an external stirrer,reflux condenser, drying tube filled with Drierite, and gas inlet tube.Anhydrous NH₃ was then bubbled through the gas inlet tube at a flow rateof approximately 3 liters/minute for 45 minutes. Dry CH₃ NH₂ was thenbubbled through the gas inlet tube until the Drierite in the drying tubeturned blue, indicating excess amine. The solution was then filtered andthe filtrate heated under vacuum to remove solvent. A yield of 25% basedon CH₃ SiCl₃ was obtained. The organosilazane is a viscous liquid withan ammoniacal odor and a molecular weight between 1,000-10,000.

The organosilazane can be polymerized to form a ceramic-precursorpolysilazane by heating at a temperature between 200°-300° C. in aninert atmosphere. The polysilazane can then be pyrolyzed to form siliconnitride----silicon carbide. Alternatively, the polysilazane can be usedto coat substrate fibers, e.g., carbon fibers, and then pyrolyzed togive a silicon nitride--silicon carbide coating on the substrate fibers.The coating increases the thermal and oxidative stability of thesubstrate fibers.

The polymerization of the particular organosilazane described formedfrom CH₃ SiCl₃, NH₃, and CH₃ NH₂ was carried out as follows.Organosilazane (1523 g) was added to a 2 liter three-neck round bottomflask equipped with a magnetic stir bar, reflux condenser, nitrogeninlet and outlet, and a thermometer. The flask was purged with nitrogenand maintained under a nitrogen atmosphere. The temperature was thenincreased slowly to 260° C. over a period of 7 hours to effectpolymerization. At the end of this period, the product polysilazane wasremoved and cooled to solidify it. The average molecular weight of thepolysilazane was 15,600, with a dispersity of 9.4. Elemental analysisresults were in agreement with those calculated for a fully-crosslinkedpolymer: calculated for CH₃ Si[(NH)_(x) (NCH₃)_(y) ]₁.5 where x=0.04,y=0.96: C-30.9; H-9.5; N-25.5; Si-34.0; found: C-31.3; H-8.6; N-24.1;Si-30.2.

The above-described polysilazane was pyrolyzed to form siliconnitride--silicon carbide by placing a small sample (4.2 g) of thepolymer in a ceramic crucible and loading the crucible in a furnacecapable of sustaining 1200° C. for several hours. The crucible was thenheated under a nitrogen atmosphere according to the following schedule.

    ______________________________________                                        Temperature     Time                                                          ______________________________________                                         275° C.-400° C.                                                                2           hours                                              400° C.-760° C.                                                                2           hours                                              760° C.-1160° C.                                                               1           hour                                              1160° C. 1.5         hours                                             ______________________________________                                    

The pyrolyzed residue in the crucible was then cooled to yield 2.4 g ofsilicon nitride--silicon carbide as a foamed black solid (57% yield).

Organosilazanes can also be prepared by reacting two differenthalosilanes with two primary amine compounds in a manner similar to thatdescribed above for one halosilane. The properties of theorganosilazanes depend on the starting materials. Molecular weightincreases with increasing molecular weight of starting materials.Generally, ease of polymerization increases with increasing halogensubstitution of the halosilanes, and with an increasing ratio of highlyhalogen substituted to less highly substituted halosilanes, anddecreases with the amount of alkyl amine used. Stability of polymerizedpreceramic product is increased by the use of some alkyl amine inorganosilazane synthesis. Generally, where the halosilanes used are nothighly halogen substituted, more ammonia than alkyl amine is used, forease of polymerization; where the halosilanes used are more highlysubstituted, more alkyl amine than ammonia is used, for stability ofproduct.

The halosilane combinations are preferably chosen to maximize theceramic yield, i.e., the yield of silicon carbide and silicon nitrideobtained upon pyrolysis of a polymer prepared from the organosilazane.It is believed that maximum yields are obtained when the theoreticalnitrogen to silicon ratio in the polymer is about 1.33. This ratio iscalculated by determining the polymer composition that would result ifeach halosilane individually reacted with the amine reactants. Forexample, reacting HSiCl₃, H₂ SiCl₂, NH₃, and CH₃ NH₂ (where the moleratio of HSiCl₃ to H₂ SiCl₂ is 2:1) would yield the following:2HSi(NZ)₁.5 /H₂ Si(NZ), where Z is H (from NH₃) or CH₃ (from CH₃ NH₂).The ratio of the number of nitrogen atoms to the number of silicon atomsis 1.33. The ability to obtain this ratio depends on the functionalitiesof the halosilanes (i.e., whether the halosilanes are di-, tri-, ortetra-halosilanes), and the mole ratio of the respective halosilanes toeach other. It has been found that combinations of a trihalosilane anddihalosilane at a ratio of 2:1 or a dihalosilane and tetrahalosilane ata ratio of 2:1 will yield the desired 1.33 theoretical ratio in thepolymer.

Another way of maximizing the ceramic yield, while at the same timemaximizing the proportion of silicon nitride compared to siliconcarbide, is to maximize the degree of halogen substitution in thehalosilane reactants. More silicon nitride is produced because there isless carbon available to form the carbide. Similarly, volatileby-products of the polymerization, which decrease the ceramic yield,tend to be hydrogen rather than hydrocarbon gas, e.g., methane, havinghigher molecular weights.

Preferred combinations of halosilanes include 2:1 mole ratios of CH₃SiCl₃ and CH₃ SiHCl₂ ; CH₃ SiHCl₂ and SiCl₄ ; HSiCl₃ and CH₃ SiHCl₂ ; H₂SiCl₂ and SiCl₄ ; and HSiCl₃ and H₂ SiCl₂. The last two cominations arethe most preferred.

Other combinations can also be used. Examples are (CH₃)₂ SiCl₂ and SiCl₄; CH₃ SiCl₃ and CH₃ SiHCl₂ ; and (CH₃)₃ SiCl and SiCl₄. Neither mono-nor tetra-substituted halosilanes should be used alone. Three or evenfour different halosilanes can be used together, and, as discussedabove, two or more different primary amines can be used as well. Thereactants can be added together or sequentially. The reaction conditionsare generally the same as described above for one halosilane and twoamines.

Other embodiments are within the following claims.

I claim:
 1. A method of preparing a polysilazane comprising the stepsof:forming an organosilazane by reacting(a) a first halosilane havingthe formula RR¹ SiX₂, wherein X is F, Cl, Br, or I, and each R and R¹,independently, is H, X, or a lower alkyl group; and (b) a secondhalosilane having the formula R⁴ R⁵ R⁶ SiX, wherein X is F, Cl, Br, orI, and each R⁴, R⁵, and R⁶, independently, is H, X, or a lower alkylgroup; and (c) a first primary amine compound having the formula R² NH₂,where R² is H or a lower alkyl group to form a reaction product; and (d)reacting said reaction product with a second primary amine compoundhaving the formula R³ NH₂, wherein R³ is H if R² is a lower alkyl groupor a lower alkyl group if R² is H, to form said organosilazane; andheating said organosilazane in an inert atmosphere to form saidpolysilazane.
 2. The method of claim 1 wherein said heating is at atemperature from about 200° C. to about 300° C.
 3. The method of claim 1wherein said first halosilane has the formula RR¹ SiX₂ or RSiX₃ and eachR and R¹, independently, is H or a lower alkyl group; and said secondhalosilane has the formula SiX₄ or R⁵ R⁶ SiX₂ and each R⁴, R⁵, and R⁶,independently, is H or a lower alkyl group, provided that (i) when saidfirst halosilane is RSiX₃, said second halosilane is R⁵ R⁶ SiX₂ and themole ratio of said first halosilane to said second halosilane is about2:1; and (ii) when said first halosilane is RR¹ SiX₂, said secondhalosilane is SiX₄ and the mole ratio of said first halosilane to saidsecond halosilane is about 2:1.
 4. The method of claim 1 wherein saidfirst halosilane is a trihalosilane.
 5. The method of claim 4 whereinsaid trihalosilane has the formula CH₃ SiCl₃.
 6. The method of claim 1wherein R² is H and R³ is a lower alkyl group.
 7. The method of claim 6wherein R³ is CH₃.
 8. The method of claim 1 or 3 wherein said firsthalosilane has the formula CH₃ SiCl₃ and said second halosilane has theformula CH₃ SiHCl₂.
 9. The method of claim 1 or 3 wherein said firsthalosilane has the formula HSiCl₃ and said second halosilane has theformula H₂ SiCl₂.
 10. The method of claims 1 or 3 wherein said firsthalosilane has the formula CH₃ SiHCl₂ and said second halosilane has theformula SiCl₄.
 11. The method of claims 1 or 3 wherein said firsthalosilane has the formula H₂ SiCl₂ and said second halosilane has theformula SiCl₄.
 12. The method of claims 1 or 3 wherein said firsthalosilane has the formula HSiCl₃ and said second halosilane has theformula CH₃ SiHCl₂.
 13. The method of claim 1 wherein the composition ofsaid first and second halosilanes and the mole ratio of said halosilanesare selected to achieve in said polysilazane a theoretical nitrogen tosilicon ratio of 1.33.